Tuesday, July 8, 2014

Super-Earth in a Polar Orbit

55 Cancri e is a transiting exoplanet orbiting the Sun-like
star 55 Cancri A. The suffix “e” indicates 55 Cancri e is the 4th planet
discovered around the star. 55 Cancri e has 8 times the mass and twice the
radius of Earth, placing it in the super-Earth-mass regime. The planet circles
its host star in an unusually close-in orbit, racing around once every 17 hours
41 minutes, at an average star-planet separation distance of only 2.3 million
km. 55 Cancri e is so near to its host star that its dayside is incinerated to a
temperature of well over 2000 K, hot enough to melt most metals.

Spectroscopic observations using the HARPS-N spectrograph
were conducted during the transit of 55 Cancri e across its host star. The
observations allow the angle between the spins of the planet’s orbit and the star’s
rotation, also known as the sky-projected obliquity, to be measured via the Rossiter-McLaughlin
(RM) effect. Basically, the RM effect is a spectroscopic phenomenon that can be
observed when a planet passes in front of its host star. As a star rotates on
its axis, one half of its visible hemisphere will be seen approaching the
observer and the other half will be seen receding away. Light from the
approaching side would appear blue-shifted and light from the receding side
would appear red-shifted.

When a planet passes in front of the star, it sequentially
blocks some of the blue-shifted and red-shifted light, or vice versa. If the
planet is in front of the blue-shifted portion, the star’s apparent radial velocity
will have a positive value (i.e. the star appears to be receding) and if the
planet is in front of the red-shifted portion, the star’s apparent radial
velocity will have a negative value (i.e. the star appears to be approaching). The
way the planet blocks the star’s blue-shifted and red-shifted light can reveal its
sky-projected obliquity.

Figure 2: The solid red line shows the best fit to the
observed Rossiter-McLaughlin anomaly of 55 Cancri e. The residuals yield a
dispersion of 0.28 m/s. Bourrier & Hebrard (2014).

Figure 3: Schematic showing the view of 55 Cancri e. During
transit, 55 Cancri e (shown as a black disk) crosses mainly in front of the
blue-shifted half of the stellar disk due to its high sky-projected obliquity
of 72.4°. Bourrier & Hebrard (2014).

In the case for 55 Cancri e, the measured RM effect is
consistent with a high sky-projected obliquity of 72.4° (+12.7° / -11.5°). This
indicates that the planet is in a highly misaligned and nearly polar orbit
around its host star. Besides 55 Cancri e, the other 4 known planets around 55
Cancri A are also likely to be highly misaligned with the star’s spin axis. The
orbits of the 5 known planets around 55 Cancri A are expected to be coplanar
with one another despite being highly misaligned with the star’s spin axis.

55 Cancri A joins Kepler-56 as the two stars known to have highly
misaligned multi-planet systems. In most multi-planet systems, including our
Solar System, the planets all have orbits that are more or less coplanar with
the equatorial planes of their host stars. 55 Cancri A has a companion star,
identified as 55 Cancri B, at a distance of 1065 AU. It has been shown that the
gravitational influence of 55 Cancri B is sufficient to have altered the
alignment of the planetary system around 55 Cancri A to what is presently
observed.